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Abstract：
Members of the Orchidaceae, one of the largest families of flowering plants, evolved the crassulacean acid metabolism (CAM) photosynthesis strategy. It is thought that CAM triggers adaptive radiation into new niche spaces, yet very little is known about its origin and diversification on different continents. Here, we assess the prevalence of CAM in Dendrobium, which is one of the largest genera of flowering plants and found in a wide range of environments, from the high altitudes of the Himalayas to relatively arid habitats in Australia. Based on phylogenetic time trees, we estimated that CAM, as determined by δ 13C values less negative than -20.0‰, evolved independently at least eight times in Dendrobium. The oldest lineage appeared in the Asian clade during the middle Miocene, indicating the origin of CAM was associated with a pronounced climatic cooling that followed a period of aridity. Divergence of the four CAM lineages in the Asian clade appeared to be earlier than divergence of those in the Australasian clade. However, CAM species in the Asian clade are much less diverse (25.6%) than those in the Australasian clade (57.9%). These findings shed new light on CAM evolutionary history and the aridity levels of the paleoclimate on different continents.
摘要：
兰科的成员，最大的开花植物家族之一，进化了十字草酸代谢（CAM）光合作用策略。人们认为CAM会触发自适应辐射进入新的生态位空间，然而，人们对它在不同大陆的起源和多样化知之甚少。这里,我们评估了石斛中CAM的患病率，它是最大的开花植物属之一，存在于广泛的环境中，从喜马拉雅山的高海拔到澳大利亚相对干旱的栖息地。基于系统发育时间树，我们估计CAM,由小于-20.0‰的δ13C值确定，在石斛中独立进化至少八次。最古老的血统出现在上世纪中期的亚洲进化枝，表明CAM的起源与干旱之后明显的气候降温有关。亚洲进化枝的四个CAM谱系的分歧似乎早于澳大利亚进化枝的分歧。然而,亚洲进化枝的CAM物种多样性（25.6％）远低于澳大利亚进化枝的CAM物种（57.9％）。这些发现为不同大陆的CAM进化史和古气候的干旱水平提供了新的思路。
